An auxiliary power-supply device mounted in electric rolling stock (hereinafter simply referred to as an electric train), is a device that converts a DC voltage into a desired DC voltage or an desired AC voltage and includes a step-down chopper for controlling a DC voltage. Generally, an output voltage sensor measures the output voltage of the step-down chopper and detects whether the output voltage of the step-down chopper is overvoltage or undervoltage using the measurement value of the output voltage sensor. When a fault occurs in the output voltage sensor, the duty cycle of the step-down chopper can increase to the extent that the output voltage of the step-down chopper is in overvoltage. When the output voltage of the step-down chopper is overvoltage or undervoltage, this can adversely affect operation of a load device connected to the auxiliary power-supply device.
The voltage converter disclosed in Patent Literature 1 determines that a fault occurs in the voltage converter when an operation value utilized by controlling means for manipulating the duty cycle is greater than or equal to a predetermined upper limit value or less than or equal to a predetermined lower limit value. Patent Literature 1 also discloses an electric train equipped with the voltage converter that stops control of a semiconductor element when a fault is detected in the voltage converter.